Methods of treating electrical heating elements

ABSTRACT

METHODS OF AND APPARATUS FOR TREATING AN ELECTRIC RESISTANCE HEATING ELEMENT OF THE TYPE HAVING AN OPENENDED METALLIC SHEATH FILLED WITH COMPACTED, ELECTRIC-INSULATING, HEAT-CONDUCTIVE MATERIAL IN WHICH IS EMBEDDED AN ELECTRICAL CONDUCTOR, WHEREBY ELECTRICAL LEAKAGE BETWEEN THE METALLIC SHEATH AND THE ELECTRICAL CONDUCTOR IS REDUCED. THE PRESENT INVENTION PROVIDES FOR THE INTRODUCTION OF AMOISTURE-PROTECTIVE FLUID, SUCH AS SILICONE OIL, INTO THE HEATING ELEMENT SHEATH UNDER PRESSURE TO ACHIEVE MAXIMUM DISPERSION OF SUCH FLUID THROUGH THE COMPACTED MATERIAL.

April 10, 1973 I G. HAWK ET AL 3,726,713

METHODS OF TFEATING ELECTRICAL HEATING ELEMENTS Filed Feb. 9, 1970 5 muwww United States Patent 3,726,713 METHODS OF TREATING ELECTRICAL HEATING ELEMENTS Grover G. Hawk, North Canton, Frank A. Martin, Akron, and Tracy A. Poulson, Bowerston, Ohio, assignors to The Hoover Company, North Canton, Ohio Filed Feb. 9, 1970, Ser. No. 9,887 Claims priority, application Great Britain, May 23, 1969,

Int. Cl. B4411 N44 US. Cl. 117231 4 Claims ABSTRACT OF THE DISCLOSURE Methods of and apparatus for treating an electric resistance heating element of the type having an openended metallic sheath filled with compacted, electric-insulating, heat-conductive material in which is embedded an electrical conductor, whereby electrical leakage between the metallic sheath and the electrical conductor is reduced. The present invention provides for the introduction of a moisture-protective fluid, such as silicone oil, into the heating element sheath under pressure to achieve maximum dispersion of such fluid through the compacted material.

BACKGROUND AND SUMMARY The present invention relates to heating elements of the type comprising an outer metallic sheath within which extends an electrical conductor with an insulating material packed between it and the sheath and the principal object of the invention is to provide new and improved methods of an apparatus for reducing electrical leakage between such conductor and sheath.

It is well known practice to manufacture such elements utilizing powdered magnesium oxide as the insulating material which is packed tightly into the sheath so as to isolate the conductor both physically and electrically from the sheath. Assuming the magnesium oxide contains no moisture quite satisfactory electrical insulation is obtained. Efforts have therefore been made to seal the end or ends of the sheath through which the conductor extends against ingress of moisture in order to ensure that a given safe leakage current between the conductor and the sheath is not exceeded; however, such efforts have not been completely successful.

Bearing in mind that this type of sheathed tubular heating element is used to a great extent in die-cast or molded electric domestic appliances such as smoothing irons or frying pans Where the sheath is in electrical conducting relationship with the body of the appliance, it will be appreciated that consistent maintenance of this insulation is essential and the present invention is concerned particularly with methods of reducing the risk of current leakage due to the presence of moisture in the insulating material and also to apparatus for carrying out such methods.

It is considered that 200 microamperes is the desirable goal for the maximum acceptable leakage current. The 200 microampere minimum level has been set as a desirable goal in a number of publications and in particular or after their incorporation by molding or die-casting in an appliance. Although this fluid apparently does not form a barrier preventing moisture from entering the element, it appears that it does prevent the moisture from altering the resistance of the magnesium oxide where it is present. The difficulty with soaking, however, is that unless the soaking period is extensive, only the ends of the element become impregnated with the silicone. For example, a few drops of silicone fluid added to the open ends of a dry untreated element may reduce leakage current to some extent. However, if elements which have been given this treatment are put on test by placement in a humidity chamber which is maintained for example at relative humidity and 85 F., the leakage current can rise to 50 microamperes after one day, about microamperes after five days and to over 1,000 microamperes after seventy-five days. Clearly, in the domestic appliance industry, it is impossible to rely on an appliance being used with suflicient regularity to prevent the moisture content rising to the extent that such leakage currents could be produced.

A further difliculty which is experienced is that al though the presence of moisture in the element on a given occasion may be relatively low, energization of the heating element and a consequent rise in temperature thereof drives the contained moisture outwardly toward the relatively cold terminal ends resulting in an accumulation of moisture in the vicinity of such ends which temporarily increases the leakage current to an undesirable degree.

Other methods have been attempted to ensure that silicone fluid is introduced into the magnesium oxide insulating material to a greater degree. For example, after soaking the open ends of an element in silicone fluid,

it has been found that the migration of the silicone fluid towards the closed center of an element can be implemented by subsequently baking the element. Such a method reduces leakage currents much better than other techniques which merely introduce silicone fluid to the open ends of the element.

A satisfactory process has been developed which requires that the open ends of the elements be soaked in silicone fluid for twenty-four hours, whereafter elements (for electrical irons) are baked for eight hours at 360 F., whereas elements for frying plans are baked for forty-seven hours at 360 F. These elements are then pressure die-cast into the iron sole plates or the bodies of the frying pans. This die-casting process does not destroy the satisfactory impregnation and tests after the treatment referred to above have indicated that the maximum leakage current, even after seventy-five hours of subjection to the humidity test, will not rise to above 30 microamperes.

'If, however, attempts are made to use this so-called soak and bake procedure on heating elements prior to their incorporation into an appliance by a gravity diecasting process, difliculty is experienced due to the fact that this process, because of the cycle times and temperatures reached, results in the silicone being boiled out of the element or decomposed by the high heat to which it has been subjected.

While it might be possible to utilize the so-called soak and bake procedure on fully manufactured appliances, e.g. frying pans with the elements molded therein, this is found to be extremely inconvenient due to the fact that silicone gets on the entire surface of the pan preventing satisfactory subsequent treatments for example, the application of non-stick coatings. Apart from this, the baking period is extensive and the volume of the baking rooms would be enormous to accommodate these relatively large, substantially complete appliances.

According to the present invention, a method of at least partially impregnating a packing insulator in a sheathed heating element with a protective fluid, i.e., one which inhibits the ingress of moisture (or the effect of moisture on the insulating properties of the packing insulator) includes the introduction of such fluid into the sheath under pressure.

While it is envisaged that the process is of particular application to elements which have already been incorporated in cast or molded appliances so that the fluid is not subjected to the molding temperatures, the scope of the invention does not preclude its use on heating elements which have not yet been integrated with the appliance but which will be so integrated by a process which does not raise the temperature of the fluid to above a critical temperature. Where the fluid is a silicone fluid, it is found that this critical temperature is around 550 F. since eventual decomposition of the silicone fluid will result if it is subjected to temperatures above this for prolonged periods. Moreover, the invention is also manifestly applicable to elements which will be used in such manner that they will be neither cast nor molded into an appliance or device.

It should be stressed that the fluid which is introduced does not necessarily prevent moisture from entering an element. It is believed that the beneficial effect, for example of silicone fluid, is obtained because it coats the minute particles of the magnesium oxide used as the packing insulator so that when moisture does extend into the element, no electrical continuity is obtained between the electrical conductor and the outer sheath of the element which will convey dangerous leakage currents.

According to another aspect of the present invention, heating element impregnation apparatus comprises clamping means for holding a heating element, or a part into which said element has been cast or molded, and coupling means for disposition in sealing relation with the open end of the heating element and in communication with a source of pressurized, moisture-protective fluid to provide for injection of such fluid into the sheath open end and through the compacted material thereat. The coupling -means may, for example, be biased toward the heating element-open end by force-applying means which may comprise a fluid pressure cylinder acting through a lever mechanism. A lost-motion connection may be interposed between the fluid cylinder and the coupling means and may include a spring which normally exerts a light sealing pressure between the coupling means and the element open end, prior to application of full sealing pressure by the fluid cylinder. During such application of full sealing pressure, the lost motion is taken up and the spring is compressed.

The invention may be carried into practice in a number of ways but one specific embodiment will now be described by way of example with reference to the accompanying drawings in which:

DRAWING DESCRIPTION FIG. 1 is a fragmentary side elevational view, partially in section, of an electric frying pan produced in accordance with the present invention;

FIG. 2 is a fragmentary, partially in section, bottom plan view of the portion of the pan adjacent the terminal ends of its heating element, and

FIG. 3 is a generally diagrammatic illustration of apparatus in which a frying pan of the type shown in FIGS. 1 and 2 is clamped and by means of which silicone fluid is introduced under pressure into the heating element.

DETAILED DESCRIPTION FIG. 1 shows a conventional cast aluminum alloy frying pan having a base and an upstanding peripheral wall 11. Cast into the base 10 is a metal sheathed heating element, the terminal ends of which are shown in FIGS. 1 and 2. The element includes an elongated, tubular metalhe sheath 12 formed to a suitable configuration with its terminal ends in side-by-side relation. Disposed within the sheath 12 and in spaced relation with its inner periphery is a resistor conductor having terminal pins 13 at respective ends thereof protruding axially outwardly of respective sheath ends. In the usual manner, the sheath 12 is filled with a highly compacted, electric-insulating, heat-conductive material 14 (such as magnesium oxide) in which the resistor conductor is embedded.

The form of sheathed heating element is manufactured according to conventional practice as a suitably looped or otherwise configurated element which is integrated with the frying pan during the manufacture of the pan proper by means of a gravity feed die-casting process. The temperatures reached in the region of the heating element during such a die-casting process are higher than pressure injection die-casting and are too high for silicone fluid to be introduced as a protective fluid into the element prior to casting since the high temperature would cause boiling off or decomposition of the fluid and render it ineffective. Accordingly, the impregnation of the heating element with the protective silicone fluid according to this specific embodiment takes place after the heating element has been integrated into a cast frying pan.

For this purpose, after the frying pan'has been cast, the sheath ends may be trimmed flush with the adjacent surface of the cast pan to expose the terminal pins 13 and the compacted magnesium oxide 14, whereafter the frying pan is heated for 15 minutes at 400 F. This may be done by connection of the terminal pins 13 to a source of electrical energy through either a proportional input timer having a thirty second cycle and set at 50% or, alternatively, through a thermostat set at approximately 400 F. in the 15 minute period. After this step, the fryin pan is cooled to 300 F. during a 15 minute period either by connection to a constant, below normal operating voltage or by connection to normal operating voltage through a thermostat adapted to cycle at 300 F.

Thereafter, the frying pan is assembled in the apparatus shown in FIG. 3 (operation of which will later be described) and silicone fluid, or oil (for example, that sold by General Electric Company under the number SF-99), is introduced under a pressure of 4000 psi on both element ends for a minimum of 10 minutes. During this period, the pan is allowed to cool normally, such cooling, it is believed, assisting flow of fluid into the element sheath. According to present practice, the amount of fluid introduced into each element to provide satisfactory results is in the order of 0.5 gram.

After disconnecting the pan from the apparatus, the pan is suitably heated and maintained at 300 F. for approximately 15 minutes, the elfect of which is to continue to distribute the silicone fluid through the compacted magnesium oxide 14.

Finally, the pan is allowed to cool and it may thereafter be completed by attachment of suitable terminals to the projecting terminal pins 13 and by attachment of suitable legs, handles and the like which form no part of the present invention and thus are not herein shown or described.

It is found that with this method of impregnation, silicone fluid is satisfactorily introduced into substantially all the interstices of the compacted magnesium oxide and tests have shown that an element in a frying pan subjected to a moisture test of RH. and 85 F. for up to 75 days does not produce a leakage current exceeding 16 microamperes.

The apparatus shown in FIG. 3 includes a base or support 20 having upstanding portions 21 and 22 between which a frying pan P is introduced and clamped by means of a toggle clamping mechanism 24 which bears against an internal surface of the side wall 11 of the pan and forces the latter against an adjustable clamping block 25 supported on the upstanding portion 22. In this position, the axially outwardly projecting terminal pins 13 of the heating element each extend vertically downwardly and are loosely received within respective bores 27 of individual sleeves 28 which are screw threaded at their lower ends into a manifold block 29 having a lateral inlet 30 for the silicone fluid. The upper ends of the sleeves 28 have annular recesses in their bores 27 which receive flexible sealing collars 32 projecting upwardly beyond the ends of the sleeves so that when the parts are in the position shown in FIG. 1, the free ends of the collars 32 abut the element sheath ends and/or the face of the metal base part of the frying pan around the two projecting terminal pins. The manifold block 29 and its associated parts thus form coupling means by which the magnesium oxide 14 at the element terminal ends is placed into sealed communication with the source of silicone fluid by means of a face seal.

It will be appreciated that in view of the high pressure employed to impregnate the compacted magnesium oxide with the silicone fluid, an equivalent force has to be applied upwardly on the seal manifold block 29 and the sleeves 28 to prevent leakage of fluid at the juncture of the collars 32 with the pan and the element sheath ends. For this purpose, the seal manifold block 29 is biased upwardly by an end 39 of a seal clamp lever 40 pivoted at 41 to the upstanding portion 21 of the base 20. The opposite end 42 of the lever is biased downwardly by a hydraulic cylinder assembly 44 mounted vertically on the upstanding portion 21.

:Extending between recesses formed respectively in the upwardly and downwardly facing surfaces of the lever 40 and the seal manifold block 29 is a spring 48. The purpose of this spring is to resiliently bias the seal manifold block 29 upwardly to engage the seal collars 32 against the ends of the element sheath and/or the adjoining face of the fry pan when the latter is initially placed in position and prior to full application of force by the cylinder assembly 44.

In view of the foregoing, it will be apparent to those skilled in the art that we have accomplished at least the principal object of our invention and it will also be apparent to those skilled in the art that the embodiment herein described may be variously changed and modified, without departing from the spirit of the invention, and that the invention is capable of uses and has advantages not herein specifically described, hence it will be appreciated that the hereindisclosed embodiment is illustrative only and that our invention is not limited thereto.

We claim:

I. The method of reducing electrical leakage between the current conductor of an electric resistance heating element embedded in compacted, electric-insulating, heatconductive material, and an enclosing metallic sheath having an open end through which a current conductor por tion extends for connection to an electrical power circuit, the improved method which comprises heating said element and driving off any moisture contained within said element sheath, placing the open end of said element sheath and the exposed, compacted material thereat in communication with a source of moisture protective fluid while the temperature of said element is elevated,

exerting pressure upon said fluid in excess of atmospheric pressure to force the same into said sheath through its open end aforesaid and through said compacted material thereat,

6 and maintaining communication aforesaid between said clement sheath and said pressurized fluid during cooling of said element.

2. The method of claim 1 which further comprises interrupting communication between said element and said fluid source after said element has cooled,

heating said element to a temperature approximating that at which fluid was initially introduced,

and maintaining said element at such temperature for a period of time sufiicient to insure substantially complete dispersion of fluid therethrough.

3. The method of claim 1 which comprises maintaining communication between said element and said fluid pressure source for a period of time suificient to insure introduction of an adequate amount of fluid into said element sheath,

interrupting communication between said element and said fluid source following cooling of said element and introduction of an adequate amount of fluid into said element sheath,

heating said element to a temperature approximating that at which fluid was initially introduced thereinto;

and maintaining said element at such temperature for a period of time sufficient to insure substantially complete dispersion of fluid therethrough.

4. The method of claim 1 which comprises initiating introduction of silicone oil into said element sheath while the temperature of said element is elevated to approximately 300 F.,

maintaining communication between said element sheath and said oil so pressurized for approximately 10 minutes while allowing said element to cool thus insuring introduction of an adequate amount of oil into said element sheath,

interrupting communication between said element and said source of silicone oil,

heating said element to approximately 300 F., and

maintaining said element at such temperature for approximately 15 minutes to insure substantially com plete dispersion of said silicone oil therethrough.

References Cited UNITED STATES PATENTS Silicones, Reinhold Pub. Co., N.Y., 1959, p. 204.

Rochow: Chemistry Wiley & Sons, 1946, p. 70.

ALFRED L. LEAVITT, Primary Examiner I. H. NEWSOME, Assistant Examiner U.S. Cl. X.R. 11765.2, B, 161 ZA; 174-418 of the Silicones, N.Y., John 

